Engines for automotive vehicles may be controlled, such as by computer modules, to operate at varying proportions of air and fuel in their combustion mixtures. Diesel engines, and other lean-burn combustion engines, are generally operated at a higher than stoichiometric air-to-fuel mass ratio to increase their fuel combustion efficiency and to improve their fuel economy. This mode of engine operation is known as “fuel-lean.” The composition of the exhaust gas from an engine operating in a fuel-lean mode includes relatively high amounts of oxygen, water, and nitrogen oxides (mostly NO and NO2, collectively NOx). An exhaust gas with a high amount of oxygen typically comprises greater than about one percent O2 by volume and up to about ten percent O2 by volume. For example, the exhaust gas of a lean-burn diesel engine has a representative composition, by volume, of about 10% oxygen, 6% carbon dioxide, 0.1% carbon monoxide (CO), 180 ppm hydrocarbons (HC), 235 ppm NOx and the balance substantially nitrogen and water.
It is desired to reduce or convert regulated constituents, such as NOx, CO, and HC, in an engine's exhaust gas to more innocuous gases, such as carbon dioxide (CO2), nitrogen (N2), and water (H2O), before the gas is released to the ambient atmosphere. To accomplish these reactions, the exhaust gas may be passed through a treatment system where it can contact materials to promote the (1) oxidation of CO to CO2, (2) oxidation of HC to CO2 and water, and (3) reduction of NOx to N2 and water. However, the high amounts of oxygen in the exhaust gas of a diesel or lean-burn engine may inhibit the catalytic reduction of NOx to N2. But, when much of the NO is oxidized to NO2, there are selective catalytic reduction additives and reaction methods for reducing much of the NO2 to N2 in the exhaust gas.
The exhaust gas treatment system of a lean-burn engine typically contains a diesel oxidation catalyst (DOC). When the exhaust gas stream is passed through the DOC it contacts a catalyst material, such as platinum, that is capable of oxidizing CO to CO2, HC to CO2 and water, and NO to NO2. The exhaust gas may then be passed through a selective catalytic reduction (SCR) system located downstream of the oxidation catalyst within the treatment system. An SCR operates by injecting a reductant material, such as ammonia or unburned fuel constituents, into the exhaust gas stream before the gas is passed over a reduction catalyst material. The reduction catalyst material is configured to reduce an amount of NO2 to N2 in the exhaust gas in the presence of the reductant additive. However, these SCR systems require a reservoir of the reductant and a dosing device to inject a controlled amount of the reductant into the exhaust gas stream. Additionally, the reductant must be injected far enough upstream of the reduction catalyst material to ensure uniform mixing in the exhaust gas.
In another approach, an engine that primarily operates in a fuel-lean mode may be controlled to briefly operate in a fuel-rich mode to increase the amount of unburned fuel constituents in the exhaust gas. When the engine is operated in the fuel-rich mode, the fuel constituents in the exhaust gas promote the reduction of NO2 to N2 in the presence of a reduction catalyst. In this treatment method, the exhaust gas is passed in contact with a combination of materials that, when combined in a treatment system, are capable of efficiently reducing NOx to N2. Such a combination is known as a Lean NOx Trap (LNT).
A conventional LNT includes a NOx oxidation catalyst, a NOx reduction catalyst, and a NOx storage material to temporarily store, or “trap,” the NOx. LNTs function under cyclical oxidizing and reducing exhaust gas conditions. And the desired cyclical exhaust gas environment is controlled by operating the associated engine in a fuel-lean mode for a major portion of an engine control cycle and in a fuel-rich mode for a minor portion of the cycle. The engine control cycle is repeated, and the desired cyclical oxidizing and reducing exhaust gas conditions are produced. However, conventional LNTs require the use of platinum (Pt) to effectively and timely oxidize NO (and CO and HC) in the exhaust gas during the fuel-lean mode of engine operation. Pt is a particularly expensive precious metal, and there is a need for a less-expensive catalyst material with equally comparable oxidation performance.